Weed Behind the Wheel

Like alcohol, marijuana impairs driving performance. The question is how much. Advocacy groups claim that marijuana affects psychomotor skills only mildly and that this doesn’t seem to play a significant role in road accidents compared to alcohol, because stoned drivers compensate for their impairment by driving more slowly and taking fewer risks (drunk drivers tend to speed and take more risks).

Interestingly, some research suggests that marijuana legalization may result in fewer traffic fatalities overall, possibly because it reduces alcohol consumption and because marijuana users, who tend to smoke at home, may be less likely to drive while impaired, compared to alcohol drinkers, who tend to drink in bars and restaurants.

But that hardly means that driving stoned is safe. A study in PLOS ONE last year found that even low THC blood concentrations decrease psychomotor skills and, as seen under special MRI imaging, increase activity in brain areas associated with “self-oriented mental activity,” which means drivers are less likely to pay attention to the road.

Also of concern is that drugged driving is on the rise, with 12 percent of drivers involved in fatal accidents testing positive for marijuana in 2010, up from 4 percent in 1999, according to a new study in the American Journal of Epidemiology.

No one knows what a safe blood THC level is (if any), how long the impairment persists after smoking, or how to best assess legal impairment limits—but as a new campaign from Colorado says, “Drive high, get a DUI.”